An 80-year-old female with one-day history of progressively increased somnolence, responsive to only tactile stimulus. Physical examination revealed an unrousable elderly female without any focal neurological deficit. Emergent magnetic resonance imaging showed no evidence of acute cerebral infarction. An electroencephalography was compatible with metabolic encephalopathy. Initial blood tests revealed elevated serum ammonia level of 204 μmol/L (normal range, 9-35 μmol/L).